Hall elements for measuring a magnetic field are very well known, and are used inter alia in current sensors, or in angular position sensors, where a magnetic field (e.g. generated by a permanent magnet) is measured at several locations of the sensor device, and is converted into an angular position, as described for example in WO9854547 (published in 1998).
The present invention is concerned with obtaining accurate measurements of a magnetic field from the Hall plates. One problem with Hall plates is that an offset is measured even in the absence of a magnetic field. Prior art solutions address this problem for example by a technique known as “spinning” or “current spinning”.
In the current spinning technique, the function of Hall plate nodes (for biasing and readout) are changed in a systematic and highly repetitive way to separate the offsets from the useful magnetic signal. With these repetitive schemes, current spinning provides a form of “chopping” adapted for use in Hall sensors. Because of transients that occur when changing the spinning phase, the maximum spinning rate fs (defined as the reciprocal of the duration of one spinning phase) is constrained. Consequently, the achievable chopping frequency (i.e. the separation in frequency of the offset from the useful signal) is also constrained. “Chopping” only separates the offsets from the useful signals, and hence in practice the unwanted offset components still have to be suppressed. This is classically done by frequency-selective removal of the offset-related components by means of filtering. In order to avoid that useful input signals are removed, this filtering needs to be done outside the frequency band occupied by the useful signals. Consequently, in practice, the useful bandwidth of the sensor signal is limited by the applied chopping frequency. Therefore, a constraint on the chopping frequency (which itself originates from a constraint on the spinning rate) translates into a constraint on the maximum sensor bandwidth.
The least residual offset is obtained when averaging four distinct phases. Such an averaging reduces the overall sensor bandwidth. When in need of high bandwidth, 4-phase spinning while averaging over only two sequential phases may be considered. However, this leads to a parasitic “half-chop-rate” tone which as the name implies is located at half of the chopping frequency. Also here, any form of filtering used for removing the “half-chop-rate” tone will also eliminate useful signals near the half-chop-rate frequency. The half-chop-rate frequency therefore marks in practice the end of the useful sensor bandwidth. Even when this “half-chop-rate” tone would be tolerated to lie inside the sensor bandwidth, it typically has a significant impact on the dynamic range.
Hall signals are very weak, and require significant amplification (e.g. a gain of 1000). A low noise (instrumentation) amplifier (LNA) may be integrated in the readout device. The LNA ideally has a bandwidth comparable to the sensor bandwidth. LNAs with a near-optimal bandwidth extending up to the chopping frequency and possibly having a high gain are known in the field, e.g. disclosed in US2017/0207761A1. When using such an LNA for Hall readout with 4-phase spinning, the above described half-chop-rate tone falls halfway of the LNA bandwidth. Since the realized gain of the LNA may be large (e.g. 1000), the half-chop-rate tone may need to be removed, or at least significantly suppressed. It is not obvious how to do this, especially when there may be an unknown input signal at the very same frequency as the half-chop-rate tone.
One approach to avoid the above problems is to apply only two-phase spinning. This however results in a larger residual offset, in magnitude comparable to the half-rate tone amplitude. So, a very similar parasitic signal is present, only at DC instead of at the half-rate frequency. As a result, similar problems arise w.r.t. the dynamic range when amplifying the signals. Furthermore, it is not clear if the larger residual offset (now at DC) can be calibrated effectively in an “on-line” way, that is in the presence of unknown external magnetic fields.
There is therefore room for improvement in methods and readout devices for reading out Hall plates.